Software-based systems, for indicating that an engine oil change in a motor vehicle is required, are known, of which some are based on algorithms which evaluate parameters such as, for instance, the mileage covered since the last oil change or the frequency of cold starts.
Alternatively, other known methods avail themselves of sensor signals which describe the physical state of the oil directly, using suitable sensors, for example, the dielectric constant of the oil or, as a far more reliable quantity, the oil viscosity being measured. In this context, from the determination of the viscosity change of the engine oil since the last oil change, a viscosity-based oil change criterion may be derived, since engine oil breakdown is usually associated with an increase in viscosity. In evaluation electronics having a connected display device in usual systems, for example, a viscosity boundary value may be stored, which is compared to measured viscosity values of the engine oil, and when they exceed it, an instruction is given to the driver as to the next due time for oil change. The physical oil parameters ascertained by these methods known from the related art are temperature-dependent without exception, so that a temperature compensation computation is necessary for the determination. Also, from unpublished Patent Application DE 100 085 47, a method for the assessment of the breakdown of engine oil is known, in which the engine oil viscosity is measured by a sensor, and in which a measuring sensor is allocated to the sensor for the oil viscosity measurement for the simultaneous determination of the oil temperature, the oil viscosity and the oil temperature being measured in the cooling phase of the engine, after it is shut off.